5.3: Classification of Neurotransmitters

Neurotransmitters play a crucial role in the communication between neurons in the autonomic nervous system. Neurons in the autonomic nervous system can be cholinergic or adrenergic depending on the neurotransmitters synthesized. Cholinergic neurons use acetylcholine as their primary neurotransmitter. This includes all the preganglionic fibers of the sympathetic and pre- and postganglionic fibers of the parasympathetic nervous systems. In addition, neurons of the somatic nervous system also use acetylcholine at the neuromuscular junction. In contrast, adrenergic neurons employ noradrenaline (also known as norepinephrine) as their main neurotransmitter. The postganglionic neurons at the sympathetic division use noradrenaline for transmission. Furthermore, the adrenal medulla releases a mixture of adrenaline and noradrenaline.

Apart from acetylcholine and noradrenaline, nonadrenergic noncholinergic (NANC) transmitters are also involved in autonomic transmission. These include ATP, neuropeptide Y (NPY), vasoactive intestinal peptides (VIP), and nitric oxide, which function at the postganglionic nerve terminal. Neurotransmitters such as γ-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT), dopamine, and substance P involved in ganglionic transmissions, are also called NANC.

Sometimes, neurotransmitters are often accompanied by co-transmitters during release. Co-transmitter helps the primary neurotransmitter in reaching remote targets and producing sustained effects. For example, acetylcholine is often associated with VIP, while acetylcholine and noradrenaline are associated with ATP. Co-transmitters are stored in separate vesicles from the neurotransmitter. Although, certain co-transmitters, such as ATP, may be stored in the same vesicle as noradrenaline. Co-transmitters can also serve as alternative transmitters, exerting trophic effects on the synapses.

Neurotransmitters are chemical messengers that transmit signals between two neurons or from neurons to effector organs. Neurons in the Autonomic Nervous System or ANS are cholinergic or adrenergic, depending on the released neurotransmitter acetylcholine or noradrenaline.

Acetylcholine facilitates signal transmission between preganglionic neurons and the autonomic ganglia of both the sympathetic and parasympathetic nervous systems.

Acetylcholine also mediates neuromuscular transmission inducing voluntary muscle contraction in the SNS.

Further, the postganglionic neurons of parasympathetic division are cholinergic in nature.

In the sympathetic division, the postganglionic neurons are adrenergic and release noradrenaline. Also, the adrenal medulla releases a mixture of adrenaline and noradrenaline.

A few NANC (Non-adrenergic non-cholinergic) transmitters, such as vasoactive intestinal peptide or VIP, also mediate autonomic transmission.

Most neurons release more than one neurotransmitter simultaneously. For example, in cholinergic neurons, VIP is co-transmitted with acetylcholine.

Each neurotransmitter stimulates specific receptors and modulates the pre- or postsynaptic effects.